Typically, vector and packaging cells are adapted to one another so that they have all the necessary elements, but they do not have overlapping elements which lead to replication-competent virus by recombination. Therefore, the sequences necessary for proper transcription of the packaging construct may be heterologous regulatory sequences derived from, for example, other human adenovirus (“Ad”) serotypes, nonhuman adenoviruses, other viruses including, but not limited to, SV40, hepatitis B virus (“HBV”), Rous Sarcoma Virus (“RSV”), cytomegalovirus (“CMV”), etc. or from higher eukaryotes such as mammals. In general, these sequences include a promoter, enhancer and polyadenylation sequences.
PER.C6 (ECACC deposit number 96022940) is an example of a cell line devoid of sequence overlap between the packaging construct and the adenoviral vector (Fallaux et al., 1998). Recombinant viruses based on subgroup C adenoviruses, such as Ad5 and Ad2, can be propagated efficiently on these packaging cells. Generation and propagation of adenoviruses from other serotypes, like subgroup B viruses, has proven to be more difficult on PER.C6 cells. However, as described in European Patent Application 00201738.2, recombinant viruses based on subgroup B virus Ad35 can be made by cotransfection of an expression construct containing the Ad35 early region-1 sequences (Ad35-E1). Furthermore, Ad35-based viruses that are deleted for E1A sequences were shown to replicate efficiently on PER.C6 cells. Thus, the E1A proteins of Ad5 complement Ad35-E1A functions, whereas, at least part of the E1B functions of Ad35 are necessary. This serotype specificity in E1B functions was recently also described for Ad7 recombinant viruses. In an attempt to generate recombinant adenoviruses derived from subgroup B virus Ad7, Abrahamsen et al. (1997) were not able to generate E1-deleted viruses on 293 cells without contamination of wild-type (wt) Ad7. Viruses that were picked after plaque purification on 293-ORF6 cells (Brough et al., 1996) were shown to have incorporated Ad7-E1B sequences by nonhomologous recombination. Thus, efficient propagation of Ad7 recombinant viruses proved possible only in the presence of Ad7-E1B expression and Ad5-E4-ORF6 expression. The E1B proteins are known to interact with cellular, as well as viral, proteins (Bridge et al., 1993; White, 1995). Possibly, the complex formed between the E1B-55K protein and E4-ORF6 which is necessary to increase mRNA export of viral proteins and to inhibit export of most cellular mRNAs, is critical and in some way serotype-specific.